A damper of the above kind is described in the specification of United Kingdom published patent application GB 2 160 296A, in which one of the said coaxial parts comprises a first flywheel mass that includes a central hub, while the other coaxial part consists of a second flywheel mass which includes a plate carrying an outer hub internally, with the outer hub partly surrounding the central hub of the other coaxial part. A circumferentially acting torsion damper couples the first mass to the said plate of the second mass.
This friction device acts in the axial direction between the first and second flywheel masses, and includes a friction ring which is in frictional contact, through one of its side faces, with a first lateral friction surface which is fixed with respect to the first mass. The arrangement also includes a Belleville ring and an application ring which are driven in rotation without any clearance, by interengagement with a set of spacer bars which are part of the torsion damper.
Thus the friction device exerts a deflecting or biassing force on these spacer bars. These latter are quite long, so that the solidity of the torsion damper may be adversely affected. In addition, the effective area of the friction surface may not be as large as is desirable, because it is interrupted locally by passages to enable the spacer bars to be secured; while, in addition, the application ring is large, and this increases its cost.
Furthermore, the friction device consists of a large number of separate components, which complicates the assembly process, while the friction device also acts permanently between the two coaxial parts of the torsion damper, exerting a frictional damping effect of constant value at the said first friction surface.
It may be desirable to have only a light damping effect at the onset of the relative angular displacement between the first and second coaxial parts, followed by a larger (and frictional) damping effect during a second phase of this angular displacement. It follows that it is desirable that the friction means should be able to act with a varying clearance.
According to the disclosure in the above mentioned United Kingdom patent specification GB 2 160 296A, this objective leads to the provision of a loose meshing arrangement between the spacer bars of the torsion damper and the application ring. This, however, also requires that the friction device should not become worn rapidly.